Evershed & Vignoles meter restoration

This old 7.5" diameter ammeter
had been virtually immersed in seawater late in its life and
the result was very heavy corrosion inside and out. The interior
was in poor shape because the glass had long since disappeared.
The movement was seized solid.

The original movement and calibration
were something like 30mA and 7.5amps respectively. Across the
terminals, inside the case, was a small shunt to give the original
full scale deflection of 7.5amps. At some point in its life this
internal shunt had been cut and a rough, home-made brass shunt
(centre-right of the picture) had been bolted externally across
the terminals to give a new full-scale reading of 75amps. The
new calibration marks had been written, not too carefully, in
blue biro.

After much discussion the owner, who
is refurbishing the boat from which the meter came, asked for
a new full-scale deflection of 150amps and, to make matters quite
tricky, wanted to fit the meter in such a way as to have it show
charge as well as discharge. This meant resetting the movement
to give a centre-zero reading. At least it started off that way,
but later had to be biased to the right to give the customer's
new requirement of a full scale "charge" reading of
150amps and a "discharge" reading, to the left, of
200Amps.

Some of the work was straightforward,
some not. The tricky bits were removing lots of rusty flakes
from within the magnet-coil space. This was not easy as the stuff,
being iron oxide, clung tenaciously to the magnet.

Once the pointer was free to move; or
at least once the coil was free to move, because the needle was
cut off before work started as it had been damaged in the past
and was a trifle wobbly; I was able to carry out basic tests
to ensure that the project was feasible.

The movement appeared to be sound and
measurements showed 30mA fsd.

Initially I planned to apply a de-ruster
after scraping off the worst of the corrosion but as work progressed,
incompatibilities in the original finish, traces of which were
still present, and the various paints I planned to use became
apparent and I finally settled for a smooth black Hammerite spray
over bare metal. This proved to be not straightforward because
when I smoothed away some blemishes prior to applying a final
coat a weakness in Hammerite showed up. One just cannot spray
onto a rubbed down finish without it ending up like the surface
of a prune and having discovered this, I removed the wrinkly
Hammerite and all traces of the original paint by disolving some
sodium hydroxide in water and applying this to the finish. After
an overnight soak the meter front panel had been stripped down
to bare metal.

Several fresh coats of Hammerite, applied
over several days, then produced a very good durable finish.
The outer case also ended up looking in good shape, with all
the over-painted heavy pitting ensuring that it looked the part
of a re-painted old meter case rather than a shiny new one.

The old scale numerals and biro marking
came off by gently rubbing with isopropyl alcohol. Retaining
as much of the original artwork; scale markings and manufacturer's
information; ensured that the dial still looked it age. Refitting
the old needle was akin to brain surgery as it is made from extremely
thin aluminium tubing, a little over 0.5mm in diameter. It was
broken off at the point where it met the coil former and due
to the dog-leg to raise it to the level of the dial meant some
creative thinking. I used a fine nylon brush bristle inserted
into the tube ends. This allowed the flexibility to position
the pointer correctly, and once held in position, a tiny quantity
of superglue was applied to the joint. This flowed into the tube
ends by capillary action and, after allowing time to set between
applications, further glue was applied until the joint was secure
and had negligible extra weight.

I checked the calibration and confirmed
that the new marking requirements were feasible by using a simple
test set, then applied new numerals with rub-on stencils.

The trickiest part of the project was
to design a suitable high current shunt, without the benefit
of a DC power supply capable of delivering more than 20amps.

I calculated the resistance of the new
shunt such that it would give the correct full scale readings,
then fashioned it from galvanised sheet steel (I won't go into
the maths here). Using a simple 20amp test set, I was able to
determine the amount of drilling and cutting to complete the
shunt. To do this I used a digital multi-meter set to milli-volts,
a dummy load made from four high power resistors, and a 15 volt
20amp PSU. Once the shunt had been finished I was able to get
the correct number of milli-volts across the meter terminals
when a current of some 20amps flowed through the circuit. Because
the scale markings are tailored to the original meter movement,
which was untouched, it should be just as accurate as it was
previously when used with the smaller shunts. True there will
be a little inaccuracy because the new shunt will heat up slightly,
and therefore change its resistance when current passes through
it, but hopefully this will be small.

Below is the meter, repainted,
and with its newly marked non-standard dial

The original meter had been calibrated
to 7.5amps and later modified to 75amps but I was unhappy about
merely making a beefier shunt without changing the method of
fitting it. The entire ship's 12-volt supply would be passing
through the meter terminals and these were not specially large
so I designed the new shunt so that the main power feed could
be connected across the shunt rather than the meter terminals.
This would ensure that the meter would never see more than its
rated 30mA, even in the event of loose connections at its terminals.

Above: The new shunt is shown
here before being fitted to the meter, by the smaller diameter
bolts.

Final testing will take place once
assembly is complete. This will involve the use of a car battery
capable of supplying enough amps into a dummy load to give full
scale deflection. To confirm the number of amps flowing a "standard"
shunt will be used. I have a suitable component shown here, marked
"250amps" and 75milli-volts.

These figures indicate the resistance
of the shunt is 300micro-ohms; therefore, dividing the number
of milli-volts across the shunt by 300 micro-ohms, when the meter
reads full scale, will determine the current indicated by the
meter.

If the new meter shunt is good the reading
on the milli-voltmeter will be 45, corresponding to a forward
deflection of 150amps and 60milli-volts in the reverse direction,
indicating 200amps. If the dial readings are low I can file away
some of the shunt material to increase its resistance, diverting
more current through the meter.